Recovery and dying: re-roll HP during each downtime. Save or die when reduced to zero HP.

Classes are the fighter (HD d8), magic-user (HD d4), and thief (HD d6) from B/X but interpreted as makes most sense in light of the below referenced 5E rules. Use fighter XP progression for all classes (page X6).

Turn undead is a first level spell; use B/X rules (page X5). Successful turns, or turn results that are doubles, do not expend the spell. Concentration required. If you want to play a demon hunter or cleric, make a fighter and take the turn undead spell as your first level feat. (Necromancers: substitute command undead.)

Fighters begin with proficiency in all weapons and medium armor. (Yes, medium. If you want to use heavy armor without penalty, you need to spend a feat.)

Magic-users begin with proficiency in daggers and no armor. Spell progression is from B/X (page X6). Roll three starting spells randomly from B/X or some other spell list. You can pick a spell too if that’s what you spend your first feat on. Spell casting in armor without proficiency is impossible.

Backstab is only for surprise melee attacks and deals +1d6 damage per point of proficiency. (But see also the sniper feat.) Distraction + successful stealth check = hidden. Attack from hiding = surprise.

Situational modifiers: Use 5E advantage and disadvantage.

Armor: Ascending AC and armors from 5E (PHB page 145). If you do not meet the heavy armor strength requirements, you take disadvantage on most physical tasks (rather than the speed modifier suggested by the 5E rules, because that does not really come into play unless you are counting squares). Shields: proficiency with medium armor grants the ability to use shields passively. Otherwise, a shield is just a weapon and you need to spend an action to get any defensive benefit.

Weapons: From B/X (damage on page B27). Attack bonus: apply your proficiency bonus to attacks with weapons for which you have proficiency. Finesse weapons: (dagger, stiletto, rapier, etc) use the dex bonus rather than the strength bonus for attack and damage. Initiative: group d6, highest wins.

Feats: Characters gain a feat at levels 1, 4, 7, 10, & 13. Yes, first level too. So go crazy with a spell-casting fighter or a swording magic-user. Who needs multi-classing? Or just go fighter/cleave. Choose from the following options whenever you get a feat. (Or roll if feeling oracular.)

This is a slick set up. I like what your doing with feats as potential for dilettante multiclassing in particular. Re: the one XP chart for all: I think I am too in love with the idea of thieves offsetting their subpar abilities by leveling up hella fast to embrace that like the newer editions.

I don’t think the system would break or anything if you used the XP tables from B/X. In the worst case, you end up with a slightly level unbalanced party, and that’s par for the course for me in any case. Though, as written here, I do think the thief is relatively potent, with the backstab tied to proficiency bonus and higher chance of succeeding at skill checks, plus any added abilities from feats.